1. Field of the Invention
The present invention relates to a release sheet. More specifically, it relates to a non-silicone-based release sheet which has a good releasability from a layer of a pressure sensitive adhesive and which has an excellent antistatic property and is suitably used for applications related to precision electronic devices since it is free of bleeding out of metal ions and ionic inorganic substances (hereinafter occasionally referred to as “ionic substances”) derived from an antistatic agent.
2. Description of the Related Art
In recent years, a pressure sensitive adhesive sheet is used in various forms at various stages of production steps for precision electronic devices such as ceramic condensers, hard disc drives, semiconductor equipments and the like.
In a pressure sensitive adhesive sheet used at such production steps for precision electronic devices, a silicone-based pressure sensitive adhesive is likely to cause troubles in electronic parts due to low molecular weight silicone compounds contained therein, and therefore non-silicone-based pressure sensitive adhesives, for example, acryl-based pressure sensitive adhesives, polyester-based pressure sensitive adhesives, polyurethane-based pressure sensitive adhesives and the like are usually used.
In the above non-silicone-based pressure sensitive adhesives, a release sheet comprising a layer of a release agent provided on a substrate is laminated thereon in order to protect a layer of a pressure sensitive adhesive until use.
A silicone-based release agent is often used for a layer of a release agent in a release sheet in general applications, and when used for electronic parts and the like, the electronic parts and the like are likely to bring about troubles by silicone compounds transferred to the layer of a pressure sensitive adhesive.
Accordingly, alkyd-based resins (refer to, for example, JP 1982-049685A) and long-chain alkyl-based resins (refer to, for example, JP 2002-249757 A) which are known as non-silicone-based release agents are tried to use for a layer of a release agent in a release sheet used for the applications described above.
However, when the above resins are used for a layer of a release agent, brought about is the problem that because of a high releasing strength required for releasing from a layer of a pressure sensitive adhesive, the layer of a pressure sensitive adhesive is not released from the layer of a release agent in a certain case.
Accordingly, a release sheet in which polybutadiene known as a rubber-based resin is used for a layer of a release agent and in which polyurethane is used for an undercoat layer is proposed as a non-silicone-based release sheet which has a good releasability from a layer of a pressure sensitive adhesive and which is excellent in an adhesion between a layer of a release agent and a substrate (refer to, for example, JP 2005-199586 A).
However, since both of the above materials have a high insulation property, releasing charge caused in releasing and delivering charge brought about when delivered from a roll can not be inhibited, and involved therein is the problem that troubles such as attracting of dirts, dusts and the like brought about.
A method in which an antistatic agent is added to a layer of a release agent is proposed as well in order to solve the above problem (refer to, for example, JP 1998-086289 A). However, involved in the above method is the problem that ionic substances contained in the antistatic agent bleed out to cause heavier releasing and whitening of a coated surface or pass through the pressure sensitive adhesive to bring about corrosion of internal elements in semiconductor devices and the like.